Devices for indication of the lunar phases of the type mentioned above exist for quite some time and traditionally have a moon disk on which two circles, which are yellow or else generally clear and symbolise the moon, are drawn on a dark background, whereas said substantially semi-circular aperture in the dial comprises a base which is separated into two concave portions, the entire assembly being known to the person skilled in the art. When this type of moon disk rotates beneath the aperture, the different lunar phases are displayed symbolically. In particular, the new moon is represented by the dark background when none of the clear circles appears in the aperture, the waxing moon and waning moon are represented by a C-shaped portion of the clear circle which is thicker or thinner when one of the circles is visible but is hidden in part by one of said concave portions of the aperture, and the full moon is represented by one of the circles being fully visible through the aperture.
However, this representation does not correspond to the actual appearance of the lunar phases during the entire lunation. In particular, the fact that all the waxing and waning phases of the portion visible from earth of the illuminated surface of the moon are represented merely by one, generally concave symbol does not correspond to reality. In fact, the natural appearance of the lunar phases, i.e. the shape of the portion which is visible from earth of the surface of the moon illuminated by the sun, depends, by generalizing to some extent, on the geometric constellation between three points, more specifically the location of the observer on the earth, the position of the moon relative to the earth, and the position of the sun relative to the earth and the moon. Depending on this geometric constellation, the observer can see all, only some, or else none of the half of the surface of the moon which is illuminated by the sun, disregarding those specific cases in which the moon is positioned in the earth's shadow.
During the waxing and waning phases of the portion visible from earth of the illuminated surface of the moon, this visible portion may also be D-shaped, in particular during lunation phases known as the first quarter and the last quarter, and may adopt a convex shape during lunation phases known as the waxing or waning gibbous moon. This appearance of the lunar phases is therefore not taken into account by the conventional, above-mentioned devices for indication of the lunar phases. Apart from electronic devices which take into account in a more detailed manner the actual shape of the portion which is visible from earth of the illuminated surface of the moon, the prior art does not yet appear to contain, up to now, a mechanism which makes it possible, with the aid of relatively simple means, to represent more realistically the lunation within the meaning mentioned above.